Abstract

The study of thermal biology can contribute to our understanding of climate change by highlighting the thermal constraints affecting a species and the adaptive mechanisms available. This thesis describes the thermal biology of the wingless grasshopper (Phaulacridium vittatum), to highlight the characteristics that predispose some insect species to cope with, or benefit from, a changing climate. These characteristics include having a wide distribution, broad thermal tolerances, morphological plasticity and a wide range of behavioural responses to temperature.
Phaulacridium vittatum is a widely distributed cool temperate species, which exhibits plasticity in melanism and body size. Geographic variation in these characters along a latitudinal gradient was assessed, to test whether variability in body size and melanism reflect adaptations to local thermal conditions. Selection for body size was found to be mediated by different factors in males and females of the wingless grasshopper. Female body size decreased significantly with latitude, while male body size was largest at intermediate latitudes. Geographic variation in the body size of the wingless grasshopper was best explained in terms of rainfall and radiation seasonality, rather than temperature. However, the body size of males was also strongly influenced by reflectance, suggesting that thermal fitness does play a role in determining adaptive responses to local conditions in this sex.
The intra-specific application of the thermal melanism hypothesis was tested, firstly by measuring the thermal properties of the different colour morphs in the laboratory, and then by comparing the average reflectance and spectral characteristics of populations along altitudinal gradients in Tasmania. Melanism in P. vittatum represents a gradation in colour rather than distinct colour morphs, with reflectance ranging from 2.49 to 5.65%. In unstriped grasshoppers, darker morphs warmed more rapidly than lighter morphs and reached a higher maximum temperature (lower temperature excess). No significant differences in thermal quality were found between the colour morphs of striped grasshoppers. In support of the thermal melanism hypothesis, grasshoppers were, on average, darker at higher altitudes; there were differences in the spectral properties of brightness and chroma between high and low altitudes; and temperature variables were significant influences on the average reflectance of female grasshoppers. However, the relationship between melanism and altitude was not consistent across all gradients, demonstrating the importance of habitat differences, and the benefit of studying trends across replicated gradients.
The range of thermal conditions typically available to ground-dwelling insects in a cool temperate region was quantified using field measurements of microhabitat temperatures over the period of a year. A steady state biophysical model was developed to predict the operative temperature of a grasshopper, in the absence of behaviour, under varying conditions of temperature, radiation, and wind. The heat capacity of the grasshopper was determined in a passive warming experiment, and used to develop a transient model that enabled thermoregulatory behaviour to be considered. These results were related to the preferred temperature of P. vittatum measured in thermal gradients in the laboratory, illustrating the extent to which microhabitat variability can provide a range of thermal opportunities and potentially alleviate the impacts of climate change on a small ectotherm.
The importance of melanism in insect thermoregulation was tested using theoretical and experimental approaches. The biophysical model was used to identify the extent to which melanism contributes to the heat balance of small ectotherms across a range of body sizes and reflectance. In the absence of behaviour, the limiting factor in the heat balance was found to be the degree of melanism, rather than small body size alone. However, in the absence of extreme melanic forms, when behaviour is incorporated by accounting for movement between sun and shade, slight differences in warming times may accumulate to amplify any thermal difference due to melanism.
To test whether preferred temperature is a plastic response to ambient temperature, and whether melanism affects behaviour, the preferred temperature of the colour morphs of P. vittatum was determined in a laboratory thermal gradient, before and after manipulating body colour by painting. The preferred temperature of the darkest colour form was significantly higher than that of the lightest form (with a mode of 37.5 – 40.0C, compared to 22.5 – 25.0C). Painting led to changes in preferred temperature, demonstrating that thermoregulatory behaviour through habitat selection is a plastic response to changes in the iii
thermal balance driven by melanism. The laboratory results were supported by measurements of live populations in natural situations.
Behavioural experiments were used to determine the means by which P. vittatum regulates its body temperature in natural situations and under artificial warming conditions in laboratory experiments. Common thermoregulatory behaviours were found to be timing of activity, choice of substrates with optimum surface temperatures, and specific behavioural postures such as stilting and vertical orientation. Behavioural responses were constant and covered a range of spatial scales, from several centimetres to many metres, and temporal scales from seconds to hours. Measurements of preferred temperature, upper critical temperature threshold (CTmax) and the Maximum Voluntarily Tolerated temperature (MVT) indicated that P.vittatum has a broad thermal tolerance, which is further extended by the existence of the different colour morphs.
The flexible thermal biology of P. vittatum suggests that it has the potential to cope with climate change without the need for significant adaptation or shifts in distribution. This has consequences for its status as an important agricultural pest in Australia, and highlights the potential for other generalist herbivores with similar characteristics to respond to changing climatic conditions.